The present invention relates to a method and apparatus for producing a porous metal via spray casting. The method is particularly well suited for producing a porous metal, and more particularly a porous aluminum alloy, where it is desirable to control levels of porosity within the porous product.
Porous metal, also referred to as metal foam, is known and has been in existence, in varying forms, for several decades. Porous metals typically are categorized as either having an "isolated" pore structure (having discrete, non-interconnected pores) or having an "interconnected" pore structure (comprised essentially of a lattice work of metal in a porous network).
Porous metals provide for a lightweight material possessing a unique combination of properties which makes them an attractive material for a variety of applications. The properties of a porous metal are dependent upon the density, the size, shape, and distribution of porosity, and the alloy and temper of the base material. Examples of porous metal properties, of particular commercial interest, include low density, high specific stiffness, and good energy and sound absorption capabilities. Additional properties of porous metals which can be controlled as a function of density include specific strength, electrical and thermal conductivity, vibration dampening capacity, fire and explosion resistance, electromagnetic shielding capabilities, buoyancy (isolated pore structure), and high surface area (interconnected pore structure). Another feature making porous metal materials attractive for a variety of uses is that they are readily fabricated into finished products, including the use of cutting and machining, brazing, mechanical fastening, adhesive bonding, plating, anodizing, and painting. Thus, it will be appreciated that the combination of unique properties provided by porous metals makes it an attractive material for a variety of automotive and aerospace applications, as well as, many other applications where the exhibited properties of porous metals are desirable.
Techniques for manufacturing or producing porous metals are also known and two of the most well recognized techniques include: 1) powder metallurgical techniques; and 2) molten metal processing techniques.
The powder metallurgy techniques usually begin with a mixture of a metal powder and a foaming agent, or a powdered alloy of the metal and foaming agent, being produced and solidified under pressure, then ground into a powder material. The powders are then evenly distributed on a vibrating conveyer which carry them through a heating chamber where the metal is melted and the foaming agent decomposed. (See, for example, U.S. Pat. Nos. 2,979,392 and 3,214,265). Other more conventional powder metallurgical techniques to produce porous metals include a powder and a foaming agent being mixed, compacted, and extruded at a temperature below the decomposition temperature of the foaming agent, to form a fully dense, solid extrusion. This extruded bar or rod is then heated, perhaps in a mold, to above the melting temperature of the base material to produce porous metal. (See, for example, U.S. Pat. No. 3,087,807). Processes utilizing powder technology are not desirable because they require multiple steps, such as powder atomization, collection, compaction and sintering. These factors are difficult to control. In addition, powder material requires special handling making its use more difficult and less desirable.
The molten metal processing techniques for producing a porous metal generally include adding a gas evolving compound or soluble gas to a molten metal. This results in the creation of a foam which is collected, cooled and solidified so as to form a solid of foamed metal. One difficulty with molten metal processing is the necessity of stabilizing the foam prior to solidification in order to prevent collapse of the levels of foam and to control the uniformity of the foam.
However, these techniques, as well as others, for producing porous metals all share inherent disadvantages, the most significant of which is the complexity of the production processes. Of course, this translates into higher costs for porous metal products.
Thus, there is identified a need for producing a porous metal which is less complex than present state of the art techniques, enabling manufacturing and production costs to be lowered so as to increase uses and acceptability of porous metals as a viable material. The method for producing a porous metal should be capable of producing a porous metal exhibiting the desired characteristics and properties which make porous metals an attractive material for numerous applications.
Spray casting, also known as spray forming, of molten metal is a well-known process for producing various types of metal products. However, spray forming techniques have not heretofore been utilized for producing porous metal with controlled levels of porosity having a desired size, shape or distribution for specific applications primarily because production of high density products is the primary goal for spray forming processes. Spray casting consists of introducing a controlled stream of molten metal into a gas-atomizing nozzle where it is impacted by high-velocity jets of gas, usually argon or nitrogen. The resulting spray of metal droplets is deposited onto a surface, such as a substrate or mold, to form a metal product. (See, for example, U.S. Pat. Nos. 4,938,278; 5,110,631; 5,143,140; and 5,154,219.)
In terms of the metal product produced, spray casting has several advantages over metal products produced by traditional ingot, roll or slab casting techniques. Spray cast metal products have extremely fine and uniform microstructure and composition. In addition, because of the rapid solidification inherent in the spray forming process, certain alloys containing high levels of alloy elements, such as aerospace aluminum alloys, that are difficult to cast in ingots or strips by roll or belt casters can be successfully made by spray casting processes.